1. The Field of the Invention
The present invention is directed generally to polymer and dye combinations for use in optical recording. More specifically, the material combinations disclosed herein are particularly useful in optical recording utilizing thermal relaxation of birefringence induced within the polymer.
2. Related Applications
This application is a continuation-in-part of copending application Ser. No. 368,072, filed Apr. 14, 1982, entitled "OPTICAL RECORDING METHOD AND APPARATUS UTILIZING BIREFRINGENT MATERIALS," now U.S. Pat. No 4,551,819, issued Nov. 5, 1985.
3. The Background of the Invention
In light of the constant improvements in the field of computer technology, video recording, and the like, it has become necessary to improve the means by which data can be rapidly and accurately recorded and accessed. Furthermore, the increasing dependence upon computer generated and recorded information and data has created problems in storing the vast amounts of information and data which are now available.
Currently, storage media typically fall into four general classifications: (1) permanent storage media on which information is recorded and then the media is rendered incapable of further recordation; (2) permanent archival storage media which is similar to a permanent storage media except that it has a much longer life; (3) postable storage media on which further information can be recorded after the initial recordation of information; and (4) erasable media. It will be readily appreciated that with the ever expanding uses for data storage, including the storage of audio visual presentations, the need for new and improved recording methods and media in each of these classifications is increasing.
In the past, magnetic tapes or discs have been the most common storage media for information and data. While magnetic storage media have the advantage of being erasable, they also have the disadvantage of being relatively low in storage density. Thus, a large volume of magnetic tape is required to store a relatively small amount of information. Accordingly, a great deal of attention has been directed toward the development of optical recording media, which are capable of recording information at a much higher density. Because most optical recording methods and media are not erasable, this type of media has been generally used for archival storage of records, documents, music, and other types of information.
In general, optical recording media employ a focused laser to induce a chemical or physical change at the point of contact between the laser and the recording media. This forms a "spot" of about one micron in diameter. In order to "write" information, the information is first converted into a digital format. Utilizing a binary format, for example, the information is converted into one of two possible symbols. These symbols may be thought of as on-off, black-white, yes-no, or 1 and 0. By coupling electrical impulses having a binary format to a light beam modulator, it is possible to reproduce the information as a set of pulses of the laser which are then directed onto the photosensitive medium, there forming white and dark spots.
In order to "read" the information, the medium is scanned by a focused laser at low power. The pattern of white and dark spots is observed to reconstruct the binary code, which in turn can be readily converted back to its original form. A low power laser is employed in this process in order to prevent further writing on the medium.
Currently, several forms of optical recording materials have proven useful to varying degrees in recording information. These classes of materials include photographic films, photoresists, photopolymers, thermo-plastics, photochromics, chalcogenide film, ablative thin films, photoferroelectric, photoconductive/electroptic, and electro-optic Each of these techniques, however, have proven to be less than satisfactory for various reasons.
The primary problem encountered in the use of photographic film, for example, is that a processing step is necessary before the data can be read. That is, the photographic film must be "developed" before it can be read. Not only does this additional processing step require additional time and facilities, but it also makes it impossible to read the information immediately after writing.
Another disadvantage of photographic films is the fact that they have low sensitivity. As a result, long laser pulses are required for the writing step, thereby slowing the writing process. Also, high costs of fabrication of photographic films, as well as additional development costs, make the use of photographic films for recording data difficult and expensive.
Photoresists are light sensitive organic materials which, upon exposure and development, form image relief patterns. The readout process measures the difference in phase between light reflected from the relief patterns in comparison to light reflected from the unexposed areas. Again, a processing step is required prior to reading the relief patterns formed by the photoresists. In addition, these materials are unstable in heat and light and have a low sensitivity. As a result, these materials are currently unacceptable as a media for recording data.
Photopolymers are organic compounds capable of undergoing photochemical reactions when irradiated with light having a certain frequency. These photochemical reactions result in products having refractive indices substantially different from those of the starting material. Thus, the recorded data is read by observing the presence or absence of localized changes of the refractive index of the medium. The primary difficulty with these materials is that if reading is done with the same laser which is used for writing, even at very low power, additional writing on the recording medium will occur. Thus, for practical use of such photopolymers, it is necessary to use a second laser wave length for reading at a frequency that is not absorbed by the recording medium.
Ablative thin films are currently the most widely used media for optical recording. Such media utilize a thin film of material capable of absorbing a coded beam of light at a desired frequency onto a substrate. The recording mechanism is essentially thermal in nature and utilizes the energy of the absorbed laser beam to either melt or actually ablate the material. The result is that dips or even holes in the film are formed. Readout can be accomplished either by means of reflector or transmission of light wherein the ablative holes or dips are used to modulate the intensity of the light beam. Currently the preferred material for use in these ablative thin films is tellurium. However, it should be noted that the use of tellurium is disadvantageous because of its high cost and its instability in air.
Thermoplastic materials are prepared by forming a multi-layer structure consisting of a substrate, such as glass or mylar; a thin conductive layer, such as gold or silver; a photoconductor, such as polyvinylcarbazole sensitized with trinitro-9-fluorenone; and a thermoplastic. The recording technique consists of forming a uniform charge on the surface of the thermoplastic so that the voltage is capacitively divided between the photoconductor and thermoplastic layers. Upon optical exposure, the photoconductor conducts at the points of illumination and thus discharges the voltage at that point. After exposure, the thermoplastic is heated so that the electrostatic forces deform the surface of the thermal plastic into a relief pattern which corresponds to the optically recorded information. It is found, however, that these materials cannot be used to achieve high density recording and, in addition, the cost of the material is prohibitively high.
A photochromic material is one which may exist in two or more relatively stable states having different optical properties. Such a material may be switched from one state to the other by photon radiation. This change of state may result in either difference absorption spectra or a difference in refractive index. This medium presently suffers from a problem of low sensitivity and thus, low accuracy in recording the desired information.
The other presently available techniques mentioned above also have various disadvantages. For example, chalcogenide materials reversibly switch between the amorphous and crystalline states upon heating, such as with a laser. However, they are currently of little practical significance because they are unstable, require high laser power for writing, must be formed as a thin layer in a manner similar to the ablative thin films, and must be read with a second laser having a different wave length than the writing laser.
Magneto-optic, photoferroelectric, and electro-optic materials utilize changes in the magnetic or electrical characteristics of a medium. However, none of these materials have yet been proven to be practical for use in optical recording of data. At the same time, photoconductive and electro-optic materials, while both photoconductive and linearly optic, may be used only for a few hours. As a result, none of these materials have been found practical for application in photorecordation of data.
From the foregoing, it can be seen that it would be highly desirable to develope a photo-optic recording medium which avoided the various disadvantages encountered in presently available photo-optic methods. Accordingly, it would be a significant advancement in the art to provide a material which was capable of use as an optical recording medium which utilized low writing energies. It would also be a significant advancement in the art to provide an optical recording medium which utilized thin films having high optical density.
It would be another advancement in the art to provide such a medium which allowed for reading with a low energy reading beam and which required no processing in which to read. It would be another advancement in the art to provide a stable recording medium capable of high density recording and involving a low signal to low noise ratio. It would also be extremely advantageous to provide such a medium which was low in cost and easy to manufacture. Such a medium, and methods for its use, are disclosed below.